This invention relates to the inhibition of metal corrosion in closed cooling systems, particularly closed systems in which aqueous coolants recirculate, such as engines, compressors and industrial process loops.
Closed recirculating cooling systems have various advantages over open cooling systems. For example, because little coolant evaporation occurs in closed systems, coolant makeup requirements are minimal. Furthermore, coolant temperatures are more easily controlled, the systems are less susceptible to biological fouling, and scale deposits are virtually eliminated. Most significantly, the absence of the continual saturation with oxygen usually encountered in open systems greatly reduces metal corrosion in closed systems. Nevertheless, closed coolant systems that are vented still suffer from metal corrosion from oxygen which enters the system at the surface of surge tanks, hot wells, and other points, or which enters as dissolved oxygen. However, the corrosion caused by dissolved oxygen is, in vented closed systems, reduced at higher operating temperatures because of the decreased solubility of oxygen at such temperatures. In completely closed systems, lacking provision for venting oxygen to the atmosphere, increased coolant temperature will increase corrosion by dissolved oxygen.
Coolants in contact with aluminum or bimetallic couples present additional problems due to greatly accelerated corrosion of aluminum above pH 8.0 and the very high concentrations of inhibitors, such as chromates, required to offset bi-metallic influences. Nitrites, nitrates, azole compounds and molybdates, individually and in certain admixtures, are known as corrosion inhibitors in aqueous compositions which contact metallic surfaces. For example, U.S. Pat. No. 4,313,837 to Vukasovich et al. discloses a water-based metal working fluid containing as a corrosion inhibitor a mixture of a molybdate and various other ingredients including a nitrite. U.S. Pat. No. 4,349,457 to Orillion discloses a resin-based coating composition containing a corrosion inhibitor comprising a mixture of borate, nitrite, and molybdate salts.
Similar corrosion inhibitor mixtures are also known for use in recirculating aqueous coolants, such as a mixture of a molybdate, a surfactant, a polyphosphate and an azole in U.S. Pat. No. 4,176,059 to Suzuki. A combination of silicate, phosphate, borate, nitrate, azole and alkali compounds in antifreeze compositions is disclosed in U.S. Pat. No. 4,455,248 to Wood. U.S. Pat. No. 3,948,792 to Watson et al. discloses corrosion and scale-softening compositions for use in engine coolants, combining azoles and nitrites with various other ingredients including nitrates, silicates, borates, pH adjusting agents and polymeric dispersants. A combination of molybdate, azole and nitrate along with other ingredients is disclosed in U.S. Pat. No. 4,149,969 to Robitaille et al. for use in coolants and a combination of nitrate or nitrite with an azole and a molybdate is disclosed in U.S. Pat. No. 4,452,715 to Hirozawa.
The foregoing patents concerned with metal corrosion inhibited coolants and molybdate as an inhibitor, require high concentrations of nitrite, nitrate, and/or molybdate, high relative proportions of other corrosion inhibiting ingredients or are limited in the cooling systems and metals which they can protect at high pH (about 8.0 to 10.5). Thus, the Hirozawa patent discloses, in Table II, a complex inhibitor mixture in an antifreeze concentrate totalling about 100-110 parts by weight, wherein the amounts of nitrate, azole and molybdate are 0.3, 0.25, and 0.01 parts by weight, respectively. The Robitaille et al patent teaches a wide variety of corrosion inhibitor systems for coolants used in open systems such as recirculating cooling towers and the like. The inhibitors include molybdate in a use-concentration of at least about 1 ppm to as high as 100 ppm, preferably about 2 to 10 ppm, in admixture with a divalent metal salt (the anion of which may be nitrate) at a cation concentration of about 0.5 to about 5 ppm and an azole such as 2-mercaptobenzothiazole, benzotriazole, and tolyltriazole at concentrations of about 1 to about 5 ppm or higher. The compositions appear to be effective for control of corrosion of ferrous, copper and copper alloy surfaces, but the patent is silent with respect to aluminum surfaces particularly under alkaline conditions.